To the female helicoidal screw of the lens barrel (outer barrel body), there is always imparted a biasing force from a plurality of looseness eliminating springs formed on the outside peripheral portion of the lens barrel (inner barrel body) which are integrally supported on the lens barrel (inner barrel body), by means of the male helicoidal screw, and the male helicoidal screw is slid while being thrusted against the female helicoidal screw.